feat(networking): enforce isolated lab egress policy

Egress enforcement is a firewall-layer control, not an application-layer
convention (correction B). HTTP_PROXY/HTTPS_PROXY configure containerd for a
clean pull path; they enforce nothing on their own.

- src/egress.js — per-environment dedicated iptables chain
  (ROOTNODE-ENV-<short-id>), built with -A (append, order-preserving) rather
  than the -I (insert-at-head) pattern an earlier design draft used, which
  would have installed rules in reverse order and let the default-deny shadow
  the proxy/DNS accepts. Required infrastructure (egress proxy, DNS) is
  explicitly permitted first, then RFC1918 ranges are dropped unconditionally,
  then a final default-deny catches the public internet. One jump rule per
  environment hooks its subnet into its chain from DOCKER-USER (the chain
  Docker guarantees it won't silently rewrite). Destroy is deterministic:
  unhook, flush, delete — no comment-grep matching.

  The ordering fix is empirically verified, not just reasoned about: the
  egress proxy's own address is itself an RFC1918 address (it's Platform-side
  infrastructure), and testing confirms it's reachable while the broader
  RFC1918 range is still blocked — exactly the scenario the original ordering
  bug would have broken.

- platform/egress-proxy/ — shared Squid instance (one per deployment, not
  per-student), destination-allowlisted (dstdomain ACL against a plain-text
  allowlist file), starting set covers docker.io/ghcr.io/gcr.io/quay.io/k8s.io.
  Proxy performs domain allowlisting; the firewall chain above is what makes
  it unbypassable, not the other way around.

  The egress proxy's address is resolved per-student-network (each network's
  Docker gateway IP, since the proxy runs with host networking and is present
  on every bridge gateway) rather than a fixed hostname — a static address
  wouldn't be reachable from more than one isolated student network at a time.
  Found while testing with two simultaneous environments.

Verified: student-to-student isolation (A cannot reach B's internal IP),
RFC1918 blocking (A cannot reach a simulated trusted-infra address), and the
proxy allowlist itself (CONNECT tunnel to an allowlisted host succeeds,
non-allowlisted gets a clean 403 from Squid).

Co-Authored-By: Claude Sonnet 5 <[email protected]>

platform/egress-proxy/Dockerfile

@@ -0,0 +1,10 @@
+FROM ubuntu:22.04
+
+RUN apt-get update && apt-get install -y --no-install-recommends squid \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY squid.conf /etc/squid/squid.conf
+COPY allowlist.txt /etc/squid/allowlist.txt
+
+EXPOSE 3128
+CMD ["squid", "-N", "-d", "1"]